When I make a demo account on the Odoo website and set up the incoming and outgoing email servers and then send a e-mail everything works fine. My files load perfect and everything is as it should be.
The e-mail from the official Odoo demo looks like this:

When I do exactly the same from my own website the images do not work.
The e-mail from my website looks like this:

I've been looking some further, at the code, and I think that the image URL's are not 'build' correctly.
Image URL from my working Odoo demo account:
https://ci3.googleusercontent.com/proxy/ISSVwvY1LwTIvbQZNg3RaCGXhaj9J-RGDN9PUWA9C9IqrsOuJBiCXOy4hwsk8OhKMU7BZd5YpwQVu3uE6Q5K5fz6Ro8BYogPtzqqVcebkhXKRGlcywaTSiy_EAoJ4AI=s0-d-e1-ft#https://test1234567.odoo.com/website/image/ir.attachment/30_10a9287/datas

Image URL from my custom website, where the image is not loaded:
https://ci4.googleusercontent.com/proxy/Ps6q0Du7Xfum6u22uLb0qfxsM2AkUFmuZmwFDOqrRdmNrJ6uxkszCXlEFrfOwsj_Ne3mwtLqrxRyI4cgfY0HHPSWWlQH9nq4PgQN3z5M-bunCEX1T1fg99LtSKTfTH0=s0-d-e1-ft#http://www.yenthevg.com:8069/website/image/ir.attachment/51_017df56/datas

Could anybody tell me why/how this happens and how I can fix this please?
None of my custom images work in my own website but they do work at the official Odoo demo..

I'm still really looking for a solution.. the googleusercontent seems to be going wrong. Looks like an URL or rights problem for the resources I guess. Any solutions or anybody else having this problem?

I can only open and view the image from the URL in my chrome, after having opened Odoo. It does not work in any other browser or anywhere else.. I'm not sure why I can't access or view the images though. The developer gives a longer URL which seems to be also combining the Google image proxy and Odoo location, as seen in this link: https://ci4.googleusercontent.com/proxy/xEdNPM1xZM7aRdtINAhS_imGdBn2kvUWed5pQYdaJi4nwOD8LOFILhRWfaHno1DD8QHMj91x-v4Utj9vQeFJxqxRWaqlyBS-FTkHlhU=s0-d-e1-ft#http://yenthevg.com:8069/mail/track/16/blank.gif?db=abc

Timo, you're right that uploading images by URL works! Sadly they want to be able to directly upload in Odoo and use those images in the mail campaigns..The problem only seems to occur when uploading on our VPS. Do you happen to know a solution for the local network to make it work?